This invention relates to a pneumatic radial tire having improved run-flat durability.
A run-flat pneumatic tire is the tire which can keep running for a predetermined distance even when an internal pressure of the tire drops drastically due to puncture and the like. To provide such run-flat performance, conventional run-flat tires are primarily directed to reduce heat generation at sidewall portions by increasing mainly rigidity of the sidewall portions as much as possible and making it more difficult for the tires to undergo flexibility. For example, Japanese patent application Kokai publication No. 64-30809 proposes a flat run-flat tire having a reinforcing liner layer which has a crescent sectional shape, is made of a rubber having a JIS--A hardness of 50 to 65 and is disposed on the inner surface of the sidewall portions of a tire having an aspect ratio of up to 55% so that its upper end overlaps with a belt layer and its lower end overlaps with a bead filler, from the aspect that the lower the aspect ratio of the tire, the higher can be made rigidity and more difficult does it become for the tire to undergo flexibility.
However, since these tires are directed to increase rigidity of the sidewall portions as much as possible so as to reduce their deflection, general excellent running performance inherent to the radial tires are reduced unavoidably. If rigidity of the sidewall portions is extremely increased in radial tires having a low aspect ratio, local strain increases particularly at the portion ranging from near the upper end of a rim flange to a belt edge of a ground contact portion because the height of the sidewall portions is small, so that run-flat durability drops.